This application claims the priority benefit of Taiwan application serial no. 91114290, filed Jun. 28, 2002.
1. Field of Invention
The present invention relates to a display device. More particularly, the present invention relates to the driving circuit of a display device.
2. Description of Related Art
Dynamic recording of documentary through film has a long history. With the invention of cathode ray tube (CTR) and broadcasting equipment, television has become an indispensable electronic device in almost every family. In the electronic industry, CRTs are also used as monitors for desktop computers. However, the CRT is now gradually being phased out due to radiation hazards and bulkiness of the CRT body that needs to house an electron gun.
Because of radiation hazards and bulkiness, flat panel displays have been developed. The types of flat panel displays now include liquid crystal display (LCD), field emission display (FED), organic light emitting diode (OLED) and plasma display panel (PDP).
Organic light emitting diode (OLED) is sometimes referred to as organic electroluminescence display (OELD). OLED is a type of self-illuminating device arranged to form a matrix of points. Each OLED is driven by a low DC current to produce light having a high luminance and contrast. The OLED also has a high operating efficiency and carries very little weight. Moreover, the OLED may emit light within a range of colors including the three primary colors red (R), green (G), blue (B) and white light. Consequently, OLED is currently the most actively developed type of flat panel display. Aside from high-resolution, lightweight, active illumination, quick response and energy saving capacity, the advantages of OLED further include a large viewing angle, good color contrast and low production cost. Currently, the OLED has many applications such as a light source at the back of a LCD or indicator panel in a mobile phone, a digital camera, a personal digital assistant (PDA) and so on.
According to the driving method, OLED may be classified into two major types, namely, a passive matrix driven type and an active matrix driven type. The passive matrix driven OLED has a simpler structure and does not use any thin film transistor (TFT). Hence, the passive matrix driven OLED is easier and less expensive to produce. However, the passive matrix driven OLED has a lower resolution and consumes a lot of electrical energy if the display area is large. On the other hand, the active matrix driven OLED is suitable for fabricating large display panels. The active matrix driven OLED panel has a wide viewing angle, illuminates brightly and responds quickly to control signals. Nevertheless, the active matrix driven OLED panel is slightly more expensive to produce.
According to the driving mode, flat panel displays may be further categorized as a voltage driven type or a current driven type. The voltage driven mode is commonly employed in a thin film transistor liquid crystal display (TFT-LCD). To produce different gray scale colors and hence a full coloration in a voltage driven TFT-LCD, different voltages are fed to respective data lines. On the other hand, the current-driven design is often employed in OLED display device. To produce different gray scale colors and hence a full coloration in a current-driven OLED display, different currents are fed to data lines.
FIG. 1 is an equivalent circuit diagram of one of the pixels inside a conventional AM-OLED display device. As shown in FIG. 1, the pixel 10 includes a driving circuit 102 and an organic light emitting diode (OLED) 104. The driving circuit 102 further includes a first thin film transistor (TFT1) 106, a capacitor (C) 108, a second thin film transistor (TFT2) 110, a third thin film transistor (TFT3) 112 and a fourth thin film transistor (TFT4) 114. The second transistor (TFT2) 110 is a driving thin film transistor that generates a driving current to light up the OLED 104. The gate of the fourth transistor (TFT4) 114 is coupled to the gate terminal of the third transistor (TFT3) 112 and a scanning voltage (Vscan). The drain terminal of the fourth transistor (TFT4) 114 is coupled to the drain terminal of the third transistor (TFT3) 112 and the drain terminal of the first transistor (TFT1) 106. The source terminal of the fourth transistor (TFT4) 114 is coupled to a terminal for receiving a data current (I). The source terminal of the third transistor (TFT3) 112 is coupled to one end of the capacitor (C) 108, the gate terminal of the first transistor (TFT1) 106 and the gate terminal of the second transistor (TFT2) 110. The source terminal of the first transistor (TFT1) 106 is coupled to the other terminal of the capacitor (C) 108, the source terminal of the second transistor (TFT2) 110 and a positive voltage terminal (VDD). The drain terminal of the second transistor (TFT2) 110 is coupled to the positive terminal of the OLED 104. The negative terminal of the OLED is connected to ground. According to FIG. 1, the driving circuit 102 has a current mirror structure. In other words, the driving current flowing through the second transistor (TFT2) 110 is determined by the data current (I). However, because of non-ideal voltage-current properties of a transistor, the driving current flowing through the second transistor (TFT2) 110 may differ from the data current (I). This may lead to the generation of an incorrect driving current and a variation of the OLED 104 luminance.
Accordingly, one object of the present invention is to provide a driving circuit for display devices. The design includes adding a biasing device to each data line. The voltage at each end of the biasing device resulting from a flow of the data current through the device is fed to a switching transistor. The voltage at each end of a light-emitting device reproduces the voltage at each end of the biasing device through a voltage coupler. Since the voltage measured at two ends of the light emitting device and the voltage measured at two ends of the biasing device are identical, the driving current flowing through the light emitting device and the data current are identical.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a driving circuit for display devices. The driving circuit drives a light-emitting device. The light-emitting device has a positive terminal and a negative terminal. The driving circuit includes a biasing device, a switching transistor, a capacitor and a voltage coupler. The biasing device has a first terminal point and a second terminal point. The first terminal point is connected to a terminal for receiving a data current while the second terminal point is connected to ground. The switching transistor has a first drain terminal, a first gate terminal and a first drain terminal. The first drain terminal is connected to the first terminal point and the first gate terminal is connected to a scanning line. The capacitor has a third terminal point and a fourth terminal point. The third terminal point is connected to the first source terminal and the fourth terminal point is connected to ground. The voltage coupler has an input terminal and an output terminal. The input terminal is connected to the first source terminal and the third terminal point and the output terminal is connected to the light-emitting device.
In one embodiment of this invention, the biasing device is an organic light emitting diode. The voltage coupler includes a driving transistor. The driving transistor has a second drain terminal, an input terminal and an output terminal. The second drain terminal is connected to a power supply. The power supply provides a voltage (VDD). The driving transistor is an N-type thin film transistor or a P-type thin film transistor. The light-emitting device is an organic light emitting diode or a high molecular weight light emitting diode. The switching transistor is an N-type thin film transistor or a P-type thin film transistor.
This invention also provides a display device that includes a plurality of pixels. Each pixel includes a switching transistor, a capacitor, a voltage coupler and a light-emitting device. The switching transistor has a first drain terminal, a first gate terminal and a first source terminal. The first drain terminal is connected to the biasing device and the first gate terminal is connected to a scanning line. The capacitor has a first terminal point and a second terminal point. The first terminal point is connected to the first source terminal and the second terminal point is connected to ground. The voltage coupler has an input terminal and an output terminal. The input terminal is connected to the first source terminal and the first terminal point. The light-emitting device has a positive terminal and a negative terminal. The positive terminal is connected to the output terminal while the negative terminal is connected to ground. The biasing device has a third terminal point and a fourth terminal point. The third terminal point is connected to a terminal for receiving data current and the first drain terminal. The fourth terminal point is connected to ground.
In brief, this invention incorporates a biasing device to each data line. The voltage at each end of the biasing device resulting from a flow of the data current through the device is fed to a switching transistor. The voltage at each end of the biasing device is transmitted without attenuation to the terminals of the light-emitting device through a voltage coupler. Since the voltage at two ends of the light-emitting device and the voltage at the terminals of the biasing device are identical, the driving current flowing through the light emitting device and the data current are identical.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.